1.Field of the Invention
The present invention relates to a character selecting mechanism for a printer, for selectively positioning characters of type wheels at the printing position on the basis of print command.
2.Description of the Prior Art
The conventional printer provided with a character selecting mechanism and a plurality of axially juxtaposed type wheels each having the types of letters, numerals and symbols (hereinafter referred to as "characters"), and ratably supported on a shaft positions the characters selectively for printing operation by means of the selector plates of a character selecting mechanism.
Such a conventional character selecting mechanism has a plurality of electromagnetic means each associated with a single type wheel. In selecting a character, the electromagnetic means is energized to drive the selector plate resting at the initial position so as to make the selector finger of the selector plate engage with a notch corresponding to the selected character of a selector ratchet to position the selected character at the printing position. On the other hand, in resetting the type wheel, the selector plate is turned by a reset cam so that the selector finger thereof is disengaged from the notch of the selector ratchet, and then the selector plate is turned further in the same direction to the initial position by a reset projection provided on the type wheel. During the type wheel resetting operation, a retaining spring applies a resilient force to the selector plate in the opposite direction so that the selector plate will not be turned excessively beyond the initial position for resetting.
Since the conventional character selecting mechanism suppresses the excessive turning of the selector plate in the resetting direction only by means of the retaining spring, the resilience of the retaining spring must be large when the character selecting mechanism is incorporated into a high-speed printer, because the selector plate is urged in the resetting direction by the comparatively large force applied thereto by the reset projection of the type wheel which rotates at a high speed. However, if increase in the resilience of individual retaining springs is small, the total increase in the resilience of the retaining springs becomes considerably large, and hence the driving force of the motor for driving the type wheels needs to be increased accordingly. Employment of a driving motor of a large capacity increases the power consumption of the printer. When the resilience of the retaining spring is excessively large, it is possible that the selector plate once returned to the initial position by the reset projection is turned in the reverse direction, namely, in the working direction, again and is caused to engage the notch of the selector ratchet to stop the type wheel unnecessarily. Accordingly, it is not desirable to increase the resilience of the retaining springs for surely retaining the selector plates at the initial position.